Added new drag hover and drop states.

Added new drop states and highlighting when tokens are dragged over drop targets.

QA:
- drag two charms to the canvas
- open the machine view
- start dragging one of the units
- the 'Create new machine' should appear in the header with new styling
- hover the unit over the drop area
- the background should go grey
- drop the unit on the header and click create in the constraints form
- click on the 'new0' machine
- start dragging the second unit
- the 'Create new machine' and 'Create new container' headers' targets should appear
- the machine and container tokens should show the 'Add to machine new0' and 'Add to container Bare metal' respectively
- hovering the unit over the headers and tokens should turn them grey
- dropping the unit on a header or token should make the drop areas disappear
By removing this you will break drop events because the dragenter event needs to be prevented to allow the drop events to fire. See https://developer.mozilla.org/en-US/docs/DragDrop/Drag_Operations#droptargets